The invention is directed to an electroacoustic transducer having a transducer membrane disposed within a cavity, by means of which the volume in the cavity is subdivided into a front membrane space and a back membrane space, with the front membrane space being defined, in part, by a mounting member or cap which is provided with acoustic transmission openings therein, with means being provided in the front membrane space for the attenuation of resonance ratio effects.
It is necessary, for increasing the intelligibility of the speech and for compensation of the frequency-dependent course of the cable attenuation, that the sensitivity of a telephone microphone steadily increase with increasing frequency in the range of 200 Hz through approximately 2500 Hz. Approximately 2.5 dB/octave is desirable.
However, the sensitivity should not exhibit any further increase in the range of 2500 Hz through 3500 Hz since feedbacks between the speaking and listening units in the handpiece of a telephone may be reduced at these frequencies.
Further, it is desirable that the frequency curve above 3500 Hz should drop steeply to avoid an interfering influence of neighboring channels in carrier-frequency communications transmission over long-distance lines, which normally involve a channel width of 4 kHz. In addition, in future PCM transmissions involving half scanning frequency, interference noise in the speech signal occasioned by the compounding is to be suppressed.
In order to achieve high sensitivity in known telephone transducers, the basic oscillation of an oscillator system (oscillator armature, synthetic membrane and coil, and bending plate) are designed with a relatively large mass m.sub.o in the middle of the telephone transmission frequency range at approximately 1 kHz. Compensation of resonance ratio effects of such basic oscillation is sought to be effected by means of a Helmholtz resonator coupled to the rear space or volume behind the membrane. (See Frequenz, Vol. 16/1962, Pages 208 through 215.)
For the increase of the transmission range up to approximately 3.5 kHz, auxiliary resonators and harmonic oscillations have been employed (see German LP Pat. No. 1,961,217), particularly the fourth partial oscillation, by means of a nodal circuit.
Since the auxiliary resonators and harmonic oscillations in part exhibit very disruptive resonance ratio effects, measures are known for the suppression of such disruptive influences by means of a special mounting of the membrane (German LP Pat. No. 1,961,217, German AS Pat. No. 1,288,146) or, by means of the specific design of the membrane per se.
Also, attempts have been made to reduce resonance effects in the travel path of the sound. Thus, it is known to arrange silk gauze behind the sound entrance holes of the mount of a microphone unit, or to dispose a piece of porous foam plastic behind such holes.
These embodiments, however, have the disadvantage that, due to the disposition of an attenuation material directly behind the sound entrance openings, such material can become dirty as a result of speaking into the unit. Consequently, over a period of time, the attenuation resistance changes and thus the frequency response and the sensitivity of the microphone likewise are changed. In addition, cleaning of the microphone unit, other than a possible replacement of the telephone is impractical. Moreover, a gluing of a silk gauze within the unit is relatively expensive. Further, such known attenuation devices disposed behind the sound openings usually are not sufficient to attenuate resonance ratio effects above 2000 Hz.